Dehydrocyclization of aliphatic C.sub.6 + compounds produces known aromatic compounds. For example, benzene and toluene are the products of n-hexane and n-heptane dehydrocyclization reactions.
Dehydrocyclization occurs in catalytic reforming which is a process in which hydrocarbon molecules are rearranged, or reformed in the presence of a catalyst. The molecular rearrangement results in an increase in the octane rating of the feedstock. Thus, during reforming low octane hydrocarbons in the gasoline boiling range are converted into high octane components by dehydrogenation of naphthenes and isomerization, dehydrocyclization and hydrocracking of paraffins.
By way of illustration, the significance of those reactions in reforming can be gleaned from a review of the following table from "Catalysis," Vol VI, P.H. Emmett (ed). Copyright 1958 by Litton Educational Publishing Company:
______________________________________ Octane Numbers of Pure Hydrocarbons Blending research octane Hydrocarbon number (clear) ______________________________________ Paraffins: n-Butane 113 n-Pentane 62 n-Hexane 19 n-Heptane 0 n-Octane -19 2-Methylhexane 41 2,2-Dimethylpentane 89 2,2,3-Trimethylbutane 113 Naphthenes (cycloparaffins): Methylcyclopentane 107 1,1-Dimethylcyclopentane 96 Cyclohexane 110 Methylcyclohexane 104 Ethylcyclohexane 43 Aromatics: Benzene 99 Toluene 124 1,3-Dimethylbenzene 145 Isopropylbenzene 132 1,3,5-Trimethylbenzene 171 ______________________________________
Naphtha reforming may also be utilized for the production of benzene, toluene, ethylbenzene, and xylene aromatics. A valuable by-product of naphtha reforming is hydrogen, which may be utilized for hydrotreating and upgrading of other hydrocarbon fractions. Generally, the molecular rearrangement of molecular components of a feed, which occurs during reforming, results in only slight, if any, changes in the boiling point of the reformate (the product of reforming), compared to that of the feed. Accordingly, reforming differs from both cracking and alkylation, both refinery processes, each of which does result in changes of boiling range of the product compared to the feed. That is, in cracking, large molecules are cracked into smaller ones; whereas, in alkylation small molecules are rebuilt into larger molecules.
The most important applications of the reforming process are as an octane upgrader and as a route to premium gasoline. Catalytic reforming is the only refining process that is capable of economically making a gasoline component having high clear research octane ratings. The charge to the reformer (straight-run, thermal, or hydrocracker naphtha) is usually available in large quantities and is of such low quality that most of it would be unsaleable without reforming.
A correlative use of catalytic reforming is in its ability to produce gasolines of acceptable volatility over a wide range of yields, through proper selection of feedstock and/or operating conditions. The refiner is thus able to vary the yield of gasoline very substantially to meet demand fluctuations. For European demand patterns, where gasoline sales are limiting and it is desired to produce as much middle distillate as practicable, the reformer can be operated on a lighter, lower volume of naphtha to minimize gasoline production while maintaining high crude runs.
Hydrogen, although often considered a by-product, is still a valuable output from the reformer. Normally, it is produced in amounts ranging from 300 to 1200 SCF/Bbl, depending on the type of feed stock and reformer operating conditions. Reformer hydrogen is used to remove unwanted contaminants from reformer feed stocks, for hydrodesulfurization of distillates, hydrocracking of heavy fractions, hydrotreating of lubes and various chemical operations. Hydrogen availability and utilization is expected to assume increasing importance as pollution restrictions lead to increasing hydroprocessing in future years.
The importance of reforming is reflected by data which indicates that finished pool gasoline is about 35% reformate in complex refineries, but can run as high as 80% in topping-reforming refineries. As lead is phased out of gasoline, more and more straight run stocks which are now blended directly into gasoline will be reformed. All current commercial reformers use a platinum containing catalyst with a hydrogen recycle stream. Within this broad definition, there are a great number of different process designs. More than 75% of the industry's reforming capacity is classified as semi-regenerative. A semi-regenerative reformer is one which runs until the catalyst is coked and then is shut down and regenerated. The time period between regenerations varies from several months to as long as 11/2 years.
Within the category of semi-regenerative reforming, a further breakdown can be made on the basis of operating pressure. Units with separator pressures of 450 psig or higher are considered high pressure units. Those with pressures of 300 psig or less are called low pressure units. Anything in between is intermediate pressure. Most of the older units are nigh pressure, while the newer designs are low or intermediate pressure. Lower pressures give better reformate yields at a given octane level.
Another type of reformer is the cyclic variety. A cyclic unit has the reactors manifolded in such a way that any reactor can be taken out of reforming service and regenerated while the other reactors are still operating. The time period between regenerations for a cyclic reactor varies from 2 to 10 days. All cyclics are low pressure.
A third type of reformer that has recently been commercialized is the continuous unit. In this type of reformer, catalyst is withdrawn from the unit during reforming, regenerated in small batches in separate regeneration facilities and then replaced in the unit. The regeneration period for continuous units is about one month. As in the case for cyclic units, all continuous units are low pressure.
Prior to about 1950 chromium oxide or molybdenum oxide supported on alumina were used to effect the two functions of a reforming catalyst. The hydrogenation-dehydrogenation function for paraffin olefin conversion during reforming is effected by the metals chromium and molybdenum and more recently platinum, rhenium, admixtures thereof and noble-metal containing trimetallic alloys. Isomerization activity was provided by the acidified alumina.
From the commercialization of platinum reforming in the middle 1950's to the late 1960's, there were no significant improvements in reforming catalysts.
In the late 1960's a dramatic breakthrough in reforming catalysts occurred. This was the introduction of the platinum-rhenium bimetallic catalysts. These catalysts have greatly improved stability compared to platinum-only catalysts. By way of background, the platinum and platinum bimetallic catalysts were generally supported on carriers.
The standard dual functional reforming catalysts exhibited high selectivity for cracking. Recently, the patent literature has started to recognize the use of platinum and non-acidic zeolite containing catalyst compositions in reforming. It has been reported that nonacidic catalysts are superior to the dual functional catalyst in selectivities, for example, those nonacidic catalysts based on zeolite X, Y, L, omega and mordenite. J. R. Bernard, PROCEEDINGS OF THE FIFTH INTERNATIONAL ZEOLITE CONFERENCE, Zeolite Conference, p686-695 (Naples 1980).
Zeolitic materials, both natural and synthetic, have been demonstrated in the past to have catalytic properties for various types of hydrocarbon conversion. Certain zeolitic materials are ordered, porous crystalline aluminosilicates having a definite crystalline structure as determined by X-ray diffraction, within which there are a large number of smaller cavities which may be interconnected by a number of still smaller channels or pores. These cavities and pores are uniform in size within a specific zeolitic material. Since the dimensions of these pores are such as to accept for adsorption molecules of certain dimensions while rejecting those of larger dimensions, these materials have come to be known as "molecular sieves" and are utilized in a variety of ways to take advantage of these properties. Such molecular sieves, both natural and synthetic, include a wide variety of positive ion-containing crystalline silicates. These silicates can be described as a rigid three-dimensional framework of SiO.sub.4 and Periodic Table Group IIIA element oxide, e.g., AlO.sub.4, in which the tetrahedra are cross-linked by the sharing of oxygen atoms whereby the ratio of the total Group IIIA element, e.g., aluminum, and silicon atoms to oxygen atoms is 1:2. The electrovalence of the tetrahedra containing the Group IIIA element, e.g., aluminum, is balanced by the inclusion in the crystal of a cation, e.g., an alkali metal or an alkaline earth metal cation. This can be expressed wherein the ratio of the Group IIA element, e.g., aluminum, to the number of various cations, such as Ca/2, Sr/2, Na, K or Li, is equal to unity. One type of cation may be exchanged either entirely or partially with another type of cation utilizing ion exchange techniques in a conventional manner. By means of such cation exchange, it has been possible to vary the properties of a given silicate by suitable selection of the cation. The spaces between the tetrahedra are occupied by molecules of water prior to dehydration.
Prior art techniques have resulted in the formation of a great variety of synthetic zeolites. Many of these zeolites have come to be designated by letter or other convenient symbols, as illustrated by zeolite Z (U.S. Pat. No. 2,882,243), zeolite X (U.S. Pat. No. 2,882,244), zeolite Y (U.S. Pat. No. 3,130,007), zeolite ZK-5 (U.S. Pat. No. 3,247,195), zeolite ZK-4 (U.S. Pat. No. 3,314,752), zeolite ZSM-5 (U.S. Pat. No. 3,702,886), zeolite ZSM-11 (U.S. Pat. No. 3,709,979), zeolite ZSM-12 (U.S. Pat. No. 3,832,449), zeolite ZSM-20 (U.S. Pat. No. 3,972,983), zeolite ZSM-35 (U.S. Pat. No. 4,016,245), and zeolite ZSM-23 (U.S. Pat. No. 4,076,842), merely to name a few.
The SiO.sub.2 /Al.sub.2 O.sub.3 ratio of a given zeolite is often variable. For example, zeolite X can be synthesized with SiO.sub.2 /Al.sub.2 O.sub.3 ratios of from 2 to 3; zeolite Y, from 3 to about 6. In some zeolites, the upper limit of the SiO.sub.2 /Al.sub.2 O.sub.3 ratio is unbounded. ZSM-5 is one such example wherein the SiO.sub.2 /Al.sub.2 O.sub.3 ratio is at least 5 and up to the limits of present analytical measurement techniques. U.S. Pat. No. 3,941,871 (Re. 29,948) discloses a porous crystalline silicate made from a reaction mixture containing no deliberately added alumina in the recipe and exhibiting the X-ray diffraction pattern characteristic of ZSM-5. U.S. Pat. Nos. 4,061,724, 4,073,865 and 4,104,294 describe crystalline silicates of varying alumina and metal content.